1. Field of the Invention
The present invention relates to a channel coding in a wireless mobile communication system, and more particularly to a method for generating codewords, which can improve decoding performance without increasing complexity.
2. Description of the Related Art
Research is actively being performed to allow the 4th generation (4G) mobile communication system to provide users with services having various levels of Quality of Service (QoS) at a high speed of about 100 Mbps. The 4G communication system targets high-speed and high-capacity communication capable of processing and transmitting various information such as images and wireless data in addition to an existing voice-based service. Accordingly, it is important to select a suitable channel coding scheme.
In contrast to wired channel environments, wireless channel environments in a mobile communication system suffer from various factors such as multi-path interference, shadowing, electric wave attenuation, time-varying noise, interference and fading. Therefore, signals transmitted from a transmission unit may include errors while passing through wireless channel environments. These errors may detrimentally affect the entire performance of the mobile communication system. Accordingly, it is necessary to use an error correcting code.
Typically, according to an error correcting code in a Code Division Multiple Access (CDMA) mobile communication system, a transmission side codes information bits and transmits coded codewords to a reception side, which receives the codewords transmitted from the transmission side, decodes the received codewords by using a decoding scheme corresponding to the coding scheme applied by the transmission side and restores the original information bits.
FIG. 1 is a block diagram schematically illustrating the structure of a known system for describing a channel coding scheme and a repetition scheme. The system includes a Forward Error Correction (FEC) unit 101, an interleaver 103, a repetition unit 105 and a modulator 107.
The FEC unit 101 receives information symbols and outputs code symbols. The code symbols are interleaved by the interleaver 103 and output to the repetition unit 105. The repetition unit 105 receives the interleaved bits, repeats the input bits by the number of times preset by the slot, and outputs predetermined information bits to the modulator 107. The modulator 107 modulates the received information bits by a predetermined modulation scheme and outputs the modulated bits.
The slot represents a bundle of code symbols included in each sub-channel. For example, when a repetition is implemented by the sub-channel and a Quadrature Phase Shift Keying (QPSK) modulation scheme is used with 48 sub-carriers per sub-channel, 96 code symbols are included in each slot. Further, when a 16 Quadrature Amplitude Modulation (QAM) is used as a modulation scheme, 192 code symbols are included in each slot.
Hereinafter, the conventional repetition scheme will be described with reference to FIGS. 2 and 3.
FIG. 2 is a diagram illustrating the general repetition scheme, which shows the conventional repetition scheme when the number of repetitions is 2 and a repetition is sequentially implemented.
Referring to FIG. 2, each reference letter, e.g. b0, b1, . . . , bN−2 or bN−1, represents an index corresponding to a code symbol. One slot includes the code symbols. The number of code symbols included in each slot may change according to modulation scheme. For example, when a QPSK modulation scheme is used, 96 code symbols may be included in each slot. Further, when a 16 QAM modulation scheme is used, 192 code symbols may be included in each slot.
The error correcting code output from the FEC unit 101 in FIG. 1 is referred to as a mother code. A codeword generated by the mother code is divided into the code symbols in each slot as described above. The code symbols are repeated by the slot. Herein, the repetition scheme used in FIG. 2 is a sequential repetition scheme. Accordingly, it can be understood that the code symbols after the first and second repetitions have the same order. Further, a constellation point of a modulation symbol determined by the code symbols is constant regardless of the number of repetitions. Hereinafter, asterisks formed by modulation symbols based on the number of sequential repetitions in FIG. 2 will be described with reference to FIG. 3.
FIG. 3 is a diagram for describing a constellation design method through a general modulation scheme, which shows a constellation for a 16 QAM modulation scheme.
Referring to FIG. 3, reference numerals represent binary bits constituting one modulation symbol. Herein, four binary bits have the Bit Error Rates (BERs) as expressed by Equations 1 and 2 below, according to the sequence. The modulation symbol represents a symbol generated by the modulator 107, and one complex modulation symbol includes one or more binary bits according to the modulation scheme.
The input binary bit sequences of the modulator 107 are divided by four bits, and are mapped to locations of m3, m2, m1 and m0 in order to form each modulation symbol. That is, when the binary bit sequences in FIG. 2 are input to the modulator 107, b0 is mapped to the location of m3, b1 is mapped to the location of m2, b2 is mapped to the location of m1 and b3 is mapped to the location of m0 in order to form one modulation symbol. Further, in the continuous bit sequence, b4 is mapped to the location of m3, b5 is mapped to the location of m2, b6 is mapped to the location of m1 and b7 is mapped to the location of m0 in order to form the next modulation symbol. Herein, the BERs of m0 and m2 may be expressed by Equation 1.
                              P                      m            ⁢                                                  ⁢            2                          =                              P                          m              ⁢                                                          ⁢              0                                =                      Q            ⁢                          {                                                                    E                    s                                                        5                    ⁢                                          N                      0                                                                                  }                                                          (        1        )            
In Equation 1, Pm2 and Pm0 represent the BERs of m0 and m2 constituting the modulation symbol, respectively, Q represents a complementary cumulative density function of a Gaussian probability variable, Es represents symbol energy and N0 represents noise power density. The BERs of m1 and m3 may be expressed by Equation 2.
                              P                      m            ⁢                                                  ⁢            3                          =                              P                          m              ⁢                                                          ⁢              1                                =                                    1              2                        ⁡                          [                                                Q                  ⁢                                      {                                                                                            E                          s                                                                          5                          ⁢                                                      N                            0                                                                                                                }                                                  +                                  Q                  ⁢                                      {                                          3                      ⁢                                                                                                    E                            s                                                                                5                            ⁢                                                          N                              0                                                                                                                                            }                                                              ]                                                          (        2        )            
In Equation 2, Pm3 and Pm1 represent the BERs of m3 and m1 constituting the modulation symbol, respectively, Q represents a complementary cumulative density function of a Gaussian probability variable, Es represents symbol energy and N0 represents noise power density.
As expressed by Equations 1 and 2, the bits constituting the modulation symbol may have different BERs according to the sequence.
In the meantime, when x>y>0, Q [x]<Q [y]. Accordingly, Equation 1 always has a value larger than that of Equation 2. In FIG. 3, the input bits mapped to the locations of m2 and m0 correspond to an inferior symbol having a high BER, and the input bits mapped to the locations of m3 and m1 correspond to a superior symbol having a low BER. In the input bit sequence (b0, b1, b2, b3 . . . ) of the modulator 107, a superior symbol and an inferior symbol are sequentially repeated.
In the repetition scheme according to the prior art as described above, after the repetition is implemented, the code symbols are mapped within each slot in the same sequence. Therefore, with the increase or decrease of the number of repetitions, the superior symbol becomes more superior and the inferior symbol becomes more inferior. This shows a gradual increase in a reliability disparity between the inferior symbol and the superior symbol, resulting in a deterioration in decoding performance.